In 3GPP Long-Term Evolution (LTE) networks, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of base stations, e.g., evolved Node-Bs (eNBs) communicating with a plurality of mobile stations referred as user equipments (UEs). Radio link monitoring (RLM) is a mechanism for a UE to monitor the quality of a downlink (DL) for determining if the radio link is good enough to continue transmission. For example, the UE shall monitor the DL quality based on cell-specific reference signal (CRS) to detect the downlink radio link quality for the serving cell. The UE shall also compare the estimated DL quality to thresholds QOUT and QIN for monitoring the downlink radio link quality of the serving cell. In addition to RLM, the UE shall consider radio link failure (RLF) to be detected upon physical layer problems based on N310/N311/T310 mechanism, random access problem indication from MAC layer, and indication from RLC layer that the maximum number of retransmission has been reached. Once RLF is detected, the UE gathers and stores RLF information and attempts RRC connection reestablishment. If such attempt fails, the UE goes back to RRC_IDLE. The UE may indicate the availability of the RLF report to eNB and report the RLF information to eNB upon request after successful RRC connection reestablishment or RRC connection setup.
In LTE Rel-10, the concept of carrier aggregation (CA) has been introduced to enhance the system throughput. With CA, two or more CCs are aggregated to support wider transmission bandwidth up to 100 MHz. A Rel-10 UE with reception and/or transmission capabilities for CA can simultaneously receive and/or transmit on multiple CCs corresponding to multiple serving cells. When CA is configured, the UE has only one RRC connection with the network. At RRC connection establishment/reestablishment or handover, one serving cell provides the NAS mobility information. At RRC connection reestablishment or handover, one serving cell provides the security input. This cell is referred to as the primary serving cell (PCELL), and other cells are referred to as the secondary serving cells (SCELLs). Depending on UE capabilities, SCELLs can be configured to form together with the PCELL as a set of serving cells.
In LTE Rel-12 and after, besides the normal eNBs, small eNBs with low transmission power and simplified protocol stacks/functionalities are introduced into E-UTRAN, which is referred to as small cell networks. The small cell architecture can be used to enhance the data throughput and to reduce the mobility signaling overhead. Instead of distributed operation, it is believed that an anchor-based architecture is a promising architecture to be operated in the small cell network. In UE anchor-based structure, a UE is housed in an eNB, which is referred to as an anchor eNB of the UE. UE anchor is UE specific, a UE anchor is a point where the Core Network connection of the UE is terminated, that does not have to be relocated when the UE moves in a local area covered by cells of multiple base-stations. UE serving cell(s) can be controlled by an eNB that is different from the anchor eNB, which is referred to as a drift eNB of the UE. When the UE is served by both anchor eNB and drift eNB, the control of the UE and the user plane functionality is split between the anchor eNB and the drift eNB.
In current LTE specification, radio link monitoring (RLM) and radio link failure (RLF) detection is only applied on PCELL, not on SCELLs. This is because in LTE Rel-10, carrier aggregation is mainly for aggregation cells in the same eNB, i.e., intra-eNB CA. In intra-eNB CA, the PCELL and SCELL share the same scheduler that is located in the same serving eNB. It is assumed that the serving eNB can detect poor link quality of SCELLs from Channel Quality Indicator (CQI) reports and/or existing RRM measurement reports from PCELL.
In UE anchor-based architecture, the anchor eNB and the drift eNB may not be physically collocated. Assume that an Xn interface is introduced for communication between the anchor eNB and the drift eNB. To avoid the backhaul delay and overhead due to information exchange between the anchor eNB and the drift eNB, and to improve flexibility and efficiency of scheduling, independent schedulers are located in the anchor eNB and the drift eNB. However, the impact of latency and overhead due to cell information exchange through the Xn interface is seriously concerned, and the channel state information of each cell should be independently report to the corresponding eNB. As a result, the anchor eNB may not be in complete control of link qualities of cells in the drift eNB. UE data transmission, UE power consumption, as well as UE user experience will be influenced if link failure occurs in the drift eNB but unknown to the anchor eNB.
A solution is sought for RLM/RLF procedures in the anchor eNB and in the drift eNB in anchor-based small cell networks.